The separation and recovery of C.sub.3 -C.sub.6 hydrocarbon dewaxing and/or deasphalting solvents from dewaxed and/or deasphalted oils is an important process consideration. It is uneconomical to simply discard the solvents after one use and similarly, in most applications their continued presence in the product oil can give rise to product quality concerns.
Such solvents have been recovered by simple distillation as well as by the simple expedient of permitting the normally gaseous solvents such as propane and butane merely vaporize. Such processes are not efficient requiring the expenditure of considerable energy to affect the distillates and, in the case of propane and butane, the recompression of the solvents before reuse is possible.
The recovery of such solvents by more energy efficient means has been explored. The use of membranes to effect the separation has generated much interest.
In U.S. Pat. No. 4,595,507 it is taught that mixtures of heavy oils and light hydrocarbons may be separated by passing the mixture through a polymeric membrane. The membrane used comprises a polymer which is capable of maintaining its integrity in the presence of hydrocarbon compounds and which has been modified by being subjected to the action of a sulfonating agent. Sulfonation agents include fuming sulfuric acid, chlorosulfonic acid, sulfur trioxides, etc. The surface or bulk modified polymer will contain a degree of sulfonation ranging from about 15% to about 50%. The separation process is affected at temperatures ranging from about ambient to about 100.degree. C. and pressures ranging from about 50 to 1000 psig. The process is described as useful for separating heavy oils from relatively light hydrocarbons which have been used as solvents in the various hydrocarbon treatment processes. The light hydrocarbon is recovered as the permeate. The membrane used may pass a wide range of pore sizes ranging from about 10 to about 500 Angstroms. The membranes are made from polymers which have the light hydrocarbon solvents in order to avoid dissolution of the membrane in the solvent. The membranes are made from polymers having solubility parameters in excess of about 9.0. Membranes which can be used include those made from polysulfone, polycarbonate, cellulose acetate, polyacrylonitrile, polyvinyl alcohol, Nylon 6,6, Nylon 8, cellulose, polybenzoimidazole, polyamide, polyimide, polytetrafluoroethylene.
U.S. Pat. No. 4,617,126 teaches that mixtures of heavy oil and light hydrocarbons may be separated by passing the mixture over a polymeric membrane which comprises a polymer capable of maintaining its integrity in the presence of hydrocarbon compounds at temperatures ranging from about ambient to about 100.degree. C. and pressures ranging from about 50 to 1000 psi. The membranes passes pore sizes ranging from about 10 to about 500 Angstroms and are cast from a solution.
The membrane is made from polymeric material which, having a different solubility parameter than the light hydrocarbon, does not dissolve in the light hydrocarbon. The polymers possess solubility parameters in excess of about 9.0 and includes polysulfone, polycarbonate, cellulose acetate, polyacrylonitrile, polyvinyl alcohol, Nylon 6,6, Nylon 8, cellulose, polybenzoimidazole, polyamide, polyimide, polytetrafluoroethylene.
The solvents recovered can include ethanol, propanol, butanol, propane, n-pentane, iso-pentane, n-hexane, n-heptane. The membranes are prepared by dissolving the polymer on a casting solvent to produce a casting solution. The casting solvent include N-methyl-2-pyrrolidone, dimethyl formamide, dimethylsulfoxide, dichloromethane, dichloroethane, chloroform, methyl cellusolve, propylene glycol or mixtures thereof such as DMF/methyl cellusolve, NMP/propylene glycol, NMP/DMSO, etc.
Interfacially polymerized membranes were initially discovered in the 1970's for use in water desalination (see "In Situ-formed Condensation Polymers for Reverse Osmosis Membranes: Second Phase", North Star Research Institute, prepared for Department of the Interior, July 1974, available from NTIS, report #PB-234 198; "Continued Evaluation of In Situ-formed Condensation Polymers for Reverse Osmosis Membranes", Midwest Research Institute, prepared for Office of Water Research and Technology, April 1976, available from NTIS, report #PB-253 193; "Interfacially Synthesized Reverse Osmosis Membrane", U.S. Pat. No. 4,277,344, July 7, 1981, assn. to Film Tec Corporation). Prior art only describes the use of these membranes for the separation of aqueous solutions by reverse osmosis.
Interfacially polymerized membranes are composed of a highly crosslinked and generally insoluble condensation polymer which is formed in situ on a micro-porous film. Most of these membranes are formed with di- or polyamines which are reacted with multi-functional iso-cyanates or acid chlorides. Amines react very readily with both of these reactants. Several of these membranes have been commercialized for water desalination purposes by companies such as UOP, Film Tec and Desalination Systems Inc. All of the commercial membranes use a polysulfone ultrafiltration membrane (0.02 to 0.1 micron pore size) for the microporous support film. Prior art does describe the use of some other microporous support films such as polyvinylchloride ultrafiltration membranes.
These membranes are formed by using the following procedures. A thin layer of a dilute solution of one component, usually an aqueous solution of the amine, is put on one side of the microporous support film. A thin layer of a dilute solution of the second component, usually in a water immersible solvent, is then put on top of the water solution layer. The order of applying the solutions can be reversed. The two components react at the water/solvent interface forming a thin (less than 1 micron thick) highly crosslinked polymer layer. This polymer layer is the active layer of the membrane at which separation occurs. Some examples of formulations mentioned in the prior art are reacting polyethylenimine with toluene diisocyanate, reacting polyethylenimine with isophthaloyl dichloride and reacting m-phenylene diamine with trimesoyl chloride.
These membranes exhibit high salt rejections from water (&gt;95%).